1. Field of the Invention
The present invention relates to a piezoelectric element, a piezoelectric actuator using the same, and an ink jet recording head.
2. Description of the Related Art
A piezoelectric element is generally constituted by lower and upper electrodes and a ferroelectric or paraelectric crystallized piezoelectric ceramics sandwiched by these electrodes. As the piezoelectric ceramics, a lead-containing piezoelectric ceramics such as lead zirconium titanate (to be referred to as “PZT” hereinafter) or a “PLZT-type” ceramics obtained by adding lanthanum to the PZT is generally used. For example, a ferroelectric using PZT is described in Applied Physics Letters, Vol. 58, No. 11, pp. 1161 to 1163, 1991.
As an application of a piezoelectric element using a film-like piezoelectric, an ink jet recording head is known. In order for a piezoelectric film used in this application to exhibit a sufficient electromechanical transducing function (pressure which encourages displacement), a film thickness of about 1 μm to 25 μm is necessary.
As a method of manufacturing the piezoelectric film, a sputtering method, a metal-organic chemical vapor deposition method (to be referred to as an MOCVD method), a chemical solution deposition method (to be referred to as a CSD method), and the like are known. The CSD method is a method which coats a solution or dispersion liquid containing metal components serving as a precursor of piezoelectric on a substrate, heats the coating film to form a metal oxide film, and calcines and crystallizes the metal oxide at a crystallizing temperature or more to form a piezoelectric film.
In general, when the metal component contained in coating liquid is a hydrolytic metallic compound, the method is called a sol-gel method. When the metal component is a thermally decomposable metal compound, the method is called a metal-organic decomposition method (to be referred to as an MOD method). In the present invention, the sol-gel method, the MOD method, and a method obtained by mixing these methods are called a “CSD” method. The CSD method has an advantage that a piezoelectric film can be easily formed.
An ink jet recording head using the PZT piezoelectric element formed by the CSD method is disclosed. For example, U.S. Pat. No. 5,265,315 and Japanese Patent Application Laid-Open No. H05-504740 disclose a method which laminates a PZT piezoelectric film having a thickness of 2 to 10 μm on a lower electrode by the CSD method and performs predetermined steps to the PZT piezoelectric film to form an ink jet recording head.
However, a conventional PZT piezoelectric element has the following problems on fatigue characteristics. When domain inversion is repeated to drive the PZT piezoelectric element, oxygen in a PZT near an electrode is short due to diffusion, a magnitude of displacement becomes small, and a leakage current increases.
In the field of a ferroelectric capacitor having a structure similar to that of the PZT piezoelectric element, in order to solve the problems, the following device is made. That is, another element is doped in a part or whole of the PZT to obtain a composition different from the PZT, thereby improving the fatigue characteristics (see Japanese Patent Application Laid-Open No. H05-190798, Japanese Patent Application Laid-Open No. H08-273436, Japanese Patent Application Laid-Open No. H10-335596, Japanese Patent Application Laid-Open No. H11-026703, and Japanese Patent Application Laid-Open No. H11-292626).
Even in the PZT piezoelectric element, the device described in the Patent Documents is applied to improve the fatigue characteristics. However, since a material which has no piezoelectricity is incorporated, the electromechanical transducing function of the entire element is deteriorated.
On the other hand, in Japanese Patent Application Laid-Open No. H06-021337, the following device is made. That is, a lead content of PZT near an electrode of a ferroelectric capacitor is purposely made excessive to improve fatigue characteristics. When this device is applied to a PZT piezoelectric element, it is expected that the electromechanical transducing function is not so deteriorated. However, an excessive lead component near the surface of the element is easily evaporated and separated. This is not preferable in terms of safety, health and environmental conservation.
In addition, Japanese Patent Application Laid-Open No. 2004-186574 discloses a piezoelectric film having a simple laminate structure constituted by a lead-containing piezoelectric film and a lead-free piezoelectric film.